Method for casting gray cast iron composition

ABSTRACT

THE PREPARATION OF CASTING OF GRAY CAST IRON HAVING REDUCED CHILL AND HARD SPOTS WITHOUT INCREASED SHRINKAGE HOLES IN WHICH THE COMPOSITION IS FORMULATED TO CONTAIN A METAL COMPONENT SUCH AS TELLURIUM, SELENIUM, BISMUTH OR MISCH-METAL, PREFERABLY IN COMBINATION WITH TITANIUM, TO CARRY THE METAL TO A LIQUID PORTION BELOW THE EUTECTIC ARREST.

United States Patent 1191 Heine et al.

11 3,762,915 1 Oct. 2, 1973 METHOD FOR CASTING GRAY CAST IRONCOMPOSITION [75] Inventors: Richard W. Heine; Carl R. Loper,

Jr., both of Madison, Wis.

[73] Assignee: Wisconsin Alumni Research Foundation, Madison, Wis.

22 Filed: July 27,1970

21 Appl. No.: 58,334

[52] US. Cl. 75/130 R, 75/123 CB, 75/123 AA, 75/123 E, 75/123 M White75/130 R X 2,978,320 4/1961 Larson 2,995,441 8/1961 Rubel 3,005,736 10/l 961 Peras 3,299,482 1/1967 Tache 75/130 R X OTHER PUBLICATIONSComstock, George F., Titanium in Iron and Steel, John Wiley and Sons,lnc., New York, 1955, p. 82-90.

Primary Examiner--L. Dewayne Rutledge Assistant Examiner-J. E. LegruAttorney-McDougall, Hersh & Scott [57] ABSTRACT The preparation ofcastings of gray cast iron having reduced chill and hard spots withoutincreased shrinkage holes in which the composition is formulated tocontain a metal component such as tellurium, selenium, bismuth orMisch-metal, preferably in combination with titanium, to carry the metalto a liquid portion below the eutectic arrest.

4 Claims, 1 Drawing Figure PATENTED Um 2W3 3.762.915

CE Carbon Equivalent 3-6 3.4 3.2

n l 1 l I l l 1 g ALLOY Cell Growf/I, Cell Growl/9 02 20900? m 2070"). fecii omzfim Ye f f 6m in? SS flun 550v? 6 o l z z fl f n 2000",; D 1 perMmuie,

/N VENTOEXS Qzchara m Heine Carl Pia ar Jr.

METHOD FOR CASTING GRAY CAST IRON COMPOSITION This invention relates togray cast irons and to compositions thereof intended to minimizeshirnkage during solidification, and to produce castings having improvedphysical and mechanical properties.

. It has been recognized that there is a tendency for the carbon that ispresent in gray cast iron to form iron carbides. This is generallyreferred to in the trade as chill.The iron carbides that form are hardand brittle and tend to produce hard and brittle castings when presentin excessive amounts.

This tendency to form iron carbides has been offset somewhat, in foundrypractice, by the addition of ingredients which tend to increase the cellcount in the cast metal, such as by the addition of ferro-silicons,titanium and the like.

In gray cast irons which are not inoculated with ferrosilicon, 4.05 to4.20 percent carbon equivalent (CE) being typical, the eutectic cellcount is normally about 1000 per square inch when the titanium contentis about 0.10 to 0.02 percent. After ferro-silicon inoculation, the cellcount may be raised to 1200 to 1600 per square inch, or even higher.Higher cell count, in the range of 1400 to 2000 per square inch, can beobtained when a titanium addition is made or residual introduced so thatthe titanium content is in the range of 0.025 to 0.05 percent. At about3.6 to 3.9 percent CE, the cell count is about 700 to 850 in lowtitanium inoculated gray cast irons with the result that chilled edgesand hard spots are secured in the products cast thereof.

This can be alleviated somewhat by raising the titanium to 0.03 to 0.05percent and inoculating with ferrosilicon to increase the cell count.

While the chill tendency is reduced, the higher cell count induced bythe presence of titanium and/or the inoculation with ferro-silicon, theamount of shrinkage in casting is undesirably increased. When thefreezing process of the cast metal ends up with shrinkage, holes form inthe portions of the castings last to solidify and unacceptable castingsresult. Thus the measures which have been taken to reduce chill and hardspots tend to increase the amount of shrinkage that takes place at theend of the freezing process thereby to raise other problems heretoforesought to be overcome by the use of risers and other procedures.

It is an object of this invention to produce gray cast iron castings andcompositions for use in same which minimize chill and the formation ofhard spots in the cast product, which provides for expansion in amountssubstantially to offset or compensate for shrinkage during the freezingprocess so that, at the end of the freezing process, the cast productwill continue to fill out the mold cavity without the need for risers orother means to fill the mold and without the development of holes in thecast product due to shrinkage in the portions last to solidify,

lt has been found that the desired results can be achieved when the graycast iron is formulated with an amount of titanium, ferro-silicon orother component to increase cell count sufficiently to minimizesolidification as carbides or phosphides thereby to reduce chill andformation of hard spots in the cast product and other components whichinsure the presence of a liquid state below the temperature for eutecticarrest so that some of the metal will solidify below the criticaltemperature of eutectic arrest for the particular composition. Suchliquid phase which remains for solidification below the criticaltemperature will freeze by a type D eutectiform graphite mechanisminstead of cell growth mechanism. The type D eutectiform graphitestructure expands upon freezing and operates in the system to offset theshrinkage which usually accumulates at the end of the freezing processand thus prevents the formation of shrinkage cavities. The eutectiformgraphite will tend to form mostly in the areas last to solidify andtherefore in higher concentrations in areas where maximum shrinkagewould otherwise occur. When the metal solidifies, the expansion of thetype D eutectiform graphite fills out the areas to compensate for theshrinkage of the type A and B cells formed during freezing at or abovethe eutectic arrest.

The amount of eutectiform graphite formed in the liquid phase remainingfor freezing below the eutectic arrest should ideally be related to theamount of type A and B cells formed so that the amount of expansion willmore or less compensate for the amount of shrink age in the final stagesof cooling. This will depend somewhat on the composition of the graycast iron from the standpoint of carbon, titanium, silicon and otheringredients present to reduce chill and hard spots. As a generalprinciple, the desired results can be achieved when the amount of type Deutectiform graphite structure that is formed is above 0.1 percent butnot more than 20 percent by volume of the gray cast iron with bestresults being secured within the range of 0.3 to 5 percent by volume oftype D eutectiform graphite structure, especially in the critical areaswhich are the last to solidify. The eutectic arrest for gray cast ironof normal composition occurs within the temperature range of 2070-21 15F. Freezing of the eutectic will begin upon cooling down through 21 15F. and will form type A and B cells. The cells that are formed duringfreezing in the upper portion of the described temperature range at andabove the eutectic arrest, will contain coarser graphite flake while thecells formed during the lower portion of the range will contain finergraphite flake. The cells all form with type A and B graphite, noteutectiform graphite.

Eutectiform graphite structure is formed at temperatures lower than 2070F., usually lower than 2055" F., by a second eutectic arrest. Theattached cooling curve shows an alloy treated to have two arrests, onewith cell growth at a higher temperature and one with eutectiformgraphite at a lower temperature. The attached cooling curves are for abase metal containing 3.36 percent carbon, 2.06 percent silicon, 0.09percent manganese, 0.12 percent phosphorus, 0 .11 percent sulphur, withthe remainder iron plus impurities. The alloy represents the base metalafter treatment with ferro-silicon in which the amounts of elements arethe same with the exception of silicon which is increased to 2.23percent.

Inoculants which increase the number of sites for cell formationgenerally raise the temperature for eutectic arrest. Thus, titanium,ferro-silicons and other metals which are added to reduce chill, such astitanium and ferro-silicon combinations marketed under the trade nameGraphidox,generally raise the temperature for eutectic arrest withcorresponding increase in the number of cells and the amount ofshrinkage that takes place during final stages of freezing.

An important concept of this invention resides in the formulation of thegray cast iron with one or more components that will cause the metal tosustain some liquid below the temperature of normal type A cell eutecticarrest so that this metal, in the liquid stage, will solidify to formeutectiform graphite structure in an amount within the range of 0.1 topercent by volume of the metal.

This can best be achieved by the formulation of the gray cast iron withtellurium and/or bismuth, rare earths, and copper. The desired resultscan be achieved when tellurium is present in the gray cast iron in anamount within the range of 0.00001 to 0.01 percent by weight andpreferably in an amount within the range of 0.0005 to 0.001 percent byweight.

Representative of other materials which are effective to carry the metalto provide a liquid phase below the critical temperature for formationof eutectiform graphite include bismuth, Misch-metal, selenium, sulphur,rare earths and copper. When use is made of selenium the amount may varyfrom a maximum of 0.1 percent ot a minimum of 0.001 percent andpreferably an amount within the range of 0.005 to 0.05 percent byweight. With bismuth, the amount may vary within the range of 0.0005 to0.02 percent and preferably within the rangeof 0.005 to 0.05 percent byweight.

An important technological advance in the art of gray cast iron residesin the ability to reduce chill and the formation of hard spots by theaddition of titaniuj, thereby to obtain the desirable effects fromtitanium additions, without the need also to accept the undesirableefi'ects previously experienced from the additions of such titanium,such as marked increase in the numbeof sites for cell formation withcorresponding increase in the amount of shrinkage. For this purpose, itis effective to make use of tellurium with titanium in the cast iron inthe ratio of onepart by weight of tellurium to 50 to 1000 parts byweighof titanium, depending somewhat upon the desired cell countreduction desired and in which the tellurium is present in the amountdescribed above. Others of the metals such as bismuth, Misch-metal,copper and selenium may be used instead of tellurium in correspondingamounts to enable the desirable effects to be secured with titaniumwithout being faced with the undesirable effects achieved when thedesired proportion of the cast metal remains in the liquid phase belowthe critical temperature for formation of eutectiform graphite duringthe final stages of solidification.

Thus, in the preferred practice of this invention, use is made of thecombination of both titanium and tellurium, or other metal component,which results in the presence of 0.1 to 15 percent by volume of a liquidphase below the critical temperature for eutectic arrest. The amount oftitanium should be at least 0.00] percent by weight but not more than0.15 percent by weight of the gray cast iron andpreferably within therange of 0.003 to 0.05 percent by weight. When the amount of titaniumexceeds 0.15 percent by weight, poor graphite shapes results and thecastings are characterized by insufficient hardness, low tensilestrength and poor machinability. When titanium is present in an amountless than 0.00l percent by'weight, it has little noticeable effect.

In practice, the tellurium can be added to the melt any time prior topouring but it is preferred to add the tellurium immediately prior topouring, as by addition to the melt in the ladle from which the metal ispoured.

Titanium can be added as a residual element in the pig iron making upthe melt, as an inoculant, such as in a titanium-bearing ferro-silicon,represented by the material marked by Vanadium Corporation as Graphidoxcontaining 10-12 percent titanium along with silicon, manganese andiron, or a similar material marketed by Foote Minerals. The titanium canbe added to the ladle with the tellurium or it can be added as aferro-titanium alloy, or it can be incorporated as part of the steelscrap.

The amounts of silicon, aluminum and calcium, when employed, are withinthe conventional amounts generally formulated into gray cast irons.

An important concept of this invention resides in the production of amaster alloy for use in addition of the titanium and tellurium in thedesired ratios and amounts to gray cast iron. Such master alloy, inaddition to containing titanium and tellurium in the ratio described,can advantageously be formulated to contain substantial amounts ofsilicon or silicon and aluminum and/or calcium. The silicon is aneffective inoculant for producing graphite and the aluminum and calciumalso have beneficial effects. In formulating the master alloy, iron canbe used as a diluent. 1

The following is representative of a master alloy embodying the featuresof this invention and which may be used for introducing titanium andtellurium into gray cast iron melts in accordance with the principles ofthis invention.

Example 1 Master alloy: Broad range Narrow range parts by weight partsby weight Aluminum 6 l2 Titanium 10 -40 12 23 Silicon 63 81 Calcium .5-2

Sodium .5 -2

Tellurium l part by weight tellurium per 25-100 parts by weight oftitanium The master alloy of Example 1 can be diluted with iron inamounts up to 50 percent by weight and preferably in amounts up to 15percent by weight.

The following are specific examples of master alloys embodying thefeatures of this invention:

Example 2 12 parts by weight aluminum 23 parts by weight titanium 63parts by weight silicon 1 part by weight calcium 1 part by weight sodium.3 part by weight tellurium Example 3 6 parts by weight aluminum 12parts by weight titanium Bl parts by weight silicon .5 part by weightcalcium .5 part by weight sodium .2 part by weight tellurium Example 412 parts by weight aluminum 23 parts by weight titanium 63 parts byweight silicon .46 part by weight tellurium l5 parts by weight ironEXAMPLE 5 Gray cast iron of convention composition is reduced to amolten state for casting by heating to a temperature of about 26002800F. The molten cast iron is tapped from the melting furnace into a ladleand before the metal is poured from the ladle, an amount of master alloyof Examples 1-4 is introduced into the molten metal to provide 20 partsper million tellurium and 0.1 percent by weight titanium.

When the added master alloy has been reduced to the molten state andmixed with the molten gray cast iron, the metal is poured from the ladleinto the molds for casting. A casting is produced which is characterizedby the presence of carbon as flake graphite and in which the casting isrelatively free of shrinkage porosity.

It will be understood that changes may be made in the details offormulation and operation without departing from the spirit of theinvention, especially as defined in the following claims.

We claim:

1. The method of casting gray cast iron with reduced chill andhard spotsand reduced shrinkage holes comprising casting molten gray cast ironcontaining titanium and a metal component which carries the metal to aliquid phase in an amount within the range of 0.1 to 20 percent byvolume below the temperature of eutectic arrest and cooling the castmetal through the temperature of 21 F. to below solidificationtemperature whereby the portions first to solidify above the eutecticarrest form into type A and B cells and the liquid portions below theeutectic arrest solidify as type Deutectiform graphite structureconcentrated in the portions last to solidify.

2. The method as claimed in claim 1 in which the metal component forcarrying the gray cast iron to a liquid phase below eutectic arrest isselected from the group consisting of tellurium, bismuth, selenium,Misch-metal, sulphur, rare earths and copper.

3. The method as claimed in claim 1 in which the metal component istellurium present in an amount within the rangeof 0.00001 to 0.01percent by weight and in which the titanium is present in the ratio ofone part by weight of tellurium to 50 to 21000 parts by weight oftitanium.

4. The method as claimed in claim 1 in which the metallic component istellurium present in an amount within the range of 0.005 to 0.001percent and in which titanium is present in an amount within the rangeof more than 0.001 percent but less than 0.15 percent by weight.

